1. Field of the Invention
The present invention relates generally to a laser angular rate sensor, and, more particularly, to an improved mirror mechanical oscillation technique, in such an angular rate sensor, for overcoming lock-in errors that occur during low angular rate sensing.
2. Description of Related Art
A laser angular rate sensor, or ring laser gyro, has two laser beams counterpropagating around a closed path by successive reflections from, typically, three or four mirrors. Upon rotation of the sensor about its sensing axis, the effective path lengths for the two beams are changed, producing a frequency difference between the counterpropagating beams which is proportional to the angular rotation rate. At low rotation rates where the frequency difference between the two laser beams would be expected to be small, it is found that the beams tend to "lock-in" or oscillate at the same frequency so that a frequency difference is not detected.
One technique, called "body dither," for eliminating lock-in has been to vibrate or dither the entire laser angular sensor about its sensing axis to raise low sensor rotation rates out of their lock-in range. Compensation for lock-in is not complete, and it may be undesirable to vibrate the entire sensor. For example, one undesirable effect of body dither when using three ring lasers in a guidance system is that vibration of one gyro may couple into another gyro.
One dither mechanism in U.S. Pat. No. 3,533,014 dithers each mirror of a three-mirror laser sinusoidally in a direction parallel to its reflective surface. The embodiment of this patent requires substantial shearing forces to dither the mirrors.
U.S. Pat. No. 4,281,930 describes a mechanism wherein all three mirrors of a three-mirror laser gyro are dithered perpendicularly to their reflective surfaces to maintain a constant cavity length. Vibrating the mirrors in this manner is easy, but unless a precise phase relation is held between the mirrors, the cavity length for the beams changes. The apparatus of this patent, therefore, needs a complex control system to eliminate the lockin phenomenon. It is difficult to remove light from the laser through mirrors covered with dither transducers.
In Oct. 1983, two U.S. patents were issued to Ljung. (U.S. Pat. No. 4,410,274 and U.S. Pat. No. 4,410,276). These patents describe a mechanism wherein two mirrors on a three-mirrored ring laser gyro are dithered one hundred and eighty degrees out-of-phase and show that transverse beam movement is produced on the third undithered mirror.
U.S. Pat. No. 4,410,274 describes a light path in an equilateral triangle. It shows that although dither perpendicularly to their surfaces of only two of the three mirrors produces backscatter shifts on all three mirrors, the amplitude of the dither cannot be adjusted to eliminate lock-in effects produced by all three mirrors. See column 2, line 47. The mechanism dithers its mirrors in a combination of directions both perpendicularly and parallelly to its mirror surfaces, simultaneously to eliminate lock-in effects from three mirrors. Accurate production of the required motion of the mirrors is very difficult. The tilted transducer stack suggested starting at column 9, line 23, tilts its attached mirror.
U.S. Pat. No. 4,410,276 pertains to a three-mirrored ring laser gyro with mirror motion only perpendicular to the mirrors. It specifies an optimum amplitude of mirror dither in an equilateral triangular laser to reduce but not eliminate mirror backscatter.
Secondly, U.S. Pat. No. 4,410,276 teaches an isosceles triangular laser of specific design with only two dithered mirrors. The teaching of equation 30 is in error, for equation 30 should read: EQU h tan.theta.sin.theta.cos.theta./h sin (90.degree.-2.theta.)=(2.405/5.520)
which reduces to (1-cos 2.theta.)/2cos2.theta.=0.4357..theta. is the angle of incidence of the laser radiation on the two mirrors at the two equal base angles of the isosceles triangle, but in an isosceles triangle the angle of incidence on the third mirror is 90 minus 2.theta., not .theta.. The value of .theta. would, thus, be changed to 28.850 degrees, and the isosceles triangle would have base angles of 57.699 degrees. The apex angle would be 64.602 degrees. With Ljung's teaching, the amplitude of mirror motion needed to eliminate lock-in would be 0.910 times the light wavelength, and such a precise dither of the light wavelength at that large excusion of the driver is extremely difficult to achieve.
The apparatus discussed in U.S. Pat. Nos. 4,281,930 (Hutchings), 4,410,274 (Ljung) and 4,410,276 (Ljung) involve mirror movement which results in pure translational motion of the light path in the cavity. Thus it is possible to find a frame of reference in which an observer views the effects of these earlier dither motions as apparent transverse motion of the mirrors parallel to their reflecting surfaces with no overall translation of the light path. In this frame of reference the dither then becomes equivalent to the apparatus discussed in U.S. Pat. No. 3,533,014 (coccoli), and thus the expression for the modulation index in terms of the amplitude of mirror motion derived there will also apply.